1. Field of the Invention
The present invention relates to rigid, closed-cell polyisocyanate-based foams and to polyol resin blends used to make such foams. In particular, the invention relates to polyol resin blends containing a modified polyester polyol, a nonionic surfactant, and a hydrocarbon blowing agent.
2. Description of the Related Art
A common process for producing polyurethane and polyisocyanurate foams requires preparing a "resin" or "B component" and subsequently mixing the resin with an isocyanate immediately prior to discharge of the final foam-generating mixture. This resin typically contains a polyol or a mixture of polyols; catalysts; silicone or other cell-stabilizing surfactants; and one or more blowing agents which vaporize due to the heat of reaction resulting in expansion of the foam. It may also contain water, as an additional blowing agent which functions by chemical generation of carbon dioxide during the reaction with isocyanate; flame retardants; and other additives.
In such a process phase stability, or resistance to separation into layers of different composition, is an important property of the resin blend. Often the resin is packaged for later sale or use, rather than being used immediately. Even if the resin is blended only by the end user, some time may elapse before it is completely consumed in the course of normal production; this elapsed time may amount to as much as several days. In either case, if separation of ingredients into discrete layers occurs, the resin will not perform correctly in use.
An additional desirable property of the resin is a viscosity that is sufficiently low to allow ease of pumping and mixing. A high resin viscosity can cause difficulties in transfer of the material, for example from storage to foam machine or from the machine holding tank to the mixing head. Excessive viscosity can also be a serious obstacle to efficient mixing with the isocyanate in the mixing head. For instance, in high-pressure impingement mixing, the efficiency of mixing may decline when the viscosity of the resin is greater than about 1000 to 2000 centipoise at the temperature of use; a viscosity of less than 1000 centipoise is to be preferred.
In an alternative process for producing polyurethane and polyisocyanurate foams, all ingredients of the resin except the blowing agent which vaporizes by heat of reaction are combined into one pre-blend. The blowing agent is then either added to the pre-blend and mixed as the combination is transferred to the final mixing head, as by the use of an inline mixer, or the blowing agent is added at the final mixing head itself. The isocyanate, or a mixture of isocyanate and blowing agent, and optionally other ingredients, are simultaneously transferred to the mixing head, where they are mixed with the resin and blowing agent and discharged to produce the polyurethane or polyisocyanurate foam. In this process, although phase stability of more than a few seconds is not necessary, it is still desirable for the pre-blend to have the property of mixing easily and uniformly with the blowing agent, and for the resulting resin blend to have a viscosity of less than 1000 centipoise to facilitate mixing with the isocyanate.
Volatile hydrocarbons such as pentane and cyclopentane are considered viable alternative blowing agents for rigid foams, but when used with existing polyols they generally result in poor phase stability and high resin viscosity. Inadequate phase stability may be addressed by introducing the blowing agent in a separate stream at or near the mix head, or by constant agitation of the pre-mixed resin in a holding tank, as described in "Hydrocarbons Provide Zero ODP and Zero GWP Insulation for Household Refrigeration" by H. Ballhaus and H. Hahn, in Proceedings of the Polyurethanes World Congress 1993, pages 33-39. However, these approaches are not useful for producing a phase-stable resin for later use, and also do not solve the problem of agitation and pumping difficulties associated with high viscosity.
U.S. Pat. No. 5,464,562 and U.S. Pat. No. 5,470,501 describe the use of certain polyoxyalkylene surfactants in combination with polyester polyols and hydrocarbon blowing agents, with improved phase stability. The stability described therein is of limited duration, however, consisting of up to 3 hours with normal pentane and 4 days with cyclopentane. In addition, these disclosures are silent with respect to undesirable high resin viscosity.